1. Field of the Invention
This invention relates to a multi channel surround sound simulation device used for home theater applications. More particularly, it relates to a device for passively decoding a surround sound encoded two-channel stereophonic soundtrack into four separate audio channels and a discreet fifth or center channel.
2. Description of Prior Art
Surround sound technology is known in the art and, until recently, most commonly used on film soundtracks for theater presentations. The surround sound process involves encoding two stereophonic channels with multiple channels of audio information by mathematical manipulation of frequency and phase information. The two channel encoded audio information is then decoded through a surround sound processor and directed to a plurality of loudspeakers thereby presenting multiple channels of audio information to a listener. Proper speaker placement allows the listener to perceive audio signals emanating from the front, rear, or side depending on the speaker placement. The ideal speaker configuration employs left and right front speakers, left and right rear speakers, and a single center front speaker. A spatial or ambience perception of sound is perceived by the listener from the rear speakers wherein out of phase signals are present. The center channel is a mono combination of the two channel input signal and used to provide frontal localization of the sound source to remove the left-right perception of the two front channels.
The most well known surround sound mode is presented by Dolby Laboratories marketed under the trademark Dolby Surround Sound. Dolby encodes video and film soundtracks for active decoding of the soundtrack thereby delivering five discreet channels of audio. Other surround sound modes are known and commonly attempt to produce sounds to simulate various concert hall settings for audio listening usually delivering four separate channels of audio information.
Recently, surround sound technology has been introduced to the home theater market wherein surround sound information is encoded on video tapes and laserdiscs. This allows individuals to experience surround sound at home, as experienced in movie theater presentations. To experience surround sound, an individual must have a means of decoding the surround sound information from a given encoded stereophonic source. Since it would be impractical for home theater listeners to purchase the expensive equipment used in theater presentations, there is a need for inexpensive systems to decode surround sound information for the home theater market.
Many attempts have been made to decode surround sound information for the home theater market thereby delivering multiple channels of audio information to a plurality of loudspeakers. One such system is seen in U. S. Pat. No. 4,953,213 to Tasaki et al., wherein the system provides surround sound simulation by providing a surround sound decoding processor, a surround mode switch circuit, and four channels of power amplification. The four channels of amplification are located along the signal path intermediate the surround sound processor and the speakers. The surround mode switch circuit selectively diverts the signal path from a third and fourth amplifier, 2.sub.3 and 2.sub.4 respectively, to deliver power output to the two rear speakers and the center speaker; the center speaker only being active in the Dolby Surround Sound mode. Although the Tasaki et al. system successfully delivers surround sound information to a plurality of speakers, the system requires four channels of amplification which can be expensive for a home audio/video enthusiast. Further, Tasaki et al. is limited to using the four channels of amplification with four of five loudspeakers in the surround sound mode. The Tasaki et al. device is unable to operate with five, three, or two channels of amplification.
Another attempt to decode surround sound encoded audio information for deliverance to a plurality of loudspeakers is seen in U.S. Pat. No. 5,265,166 to Madnick et al. The Madnick et al. system is able to deliver audio information to either two, four, or five speakers depending on the surround mode being decoded. A switch allows a user of the system to switch between the different surround sound modes. The Madnick et al. system receives two channel surround sound encoded information from a two channel amplified signal source, produces the separate channels, and disperses the decoded surround sound information to two, four, or five loudspeakers, respectively. The Madnick et al. system delivers a speaker level output to all the output terminals for direct connection with the speakers; there are no line level outputs on the Madnick et al. system. Further, the Madnick et al. system includes an inhibiting circuit to reduce high frequency information from the two rear speaker and center speaker outputs The inhibiting circuit is located within the circuit of the system intermediate the signal source input and the speaker outputs of the surround sound system. This inhibiting circuit evolved from a need to reduce high frequency information from the rear and center channels from surround sound encoded video tapes and laserdiscs. When surround sound audio first appeared in the home market, the encoded soundtracks were transferred or "dubbed" directly from the film version shown in movie theaters to the video tape or laserdisc used for home presentation. The high frequency information in the rear and center channels of surround sound encoded film soundtracks are intentionally boosted for theater presentations to compensate for sound absorption by the movie screen, carpeting, and other similar absorbing materials found within a movie theater. Therefore, there was a need for an inhibiting circuit as seen in Madnick et al.
Recently however, the film industry has compensated for the boosted high frequency information present on film soundtracks by rolling off the high frequencies during the film to video tape and laserdisc transferring process. Therefore, the inhibiting circuit of Madnick et al. is moot. Further to Madnick et al., the four or five respective output channels are driven entirely by two channels of amplification. Although two channels of amplification minimizes expenses in the Madnick et al. system, it does not allow discreet channel separation of the four or five respective output channels. Amplitude changes of the rear and center speakers are directly proportional to each other. It is common for an individual listening to a surround sound presentation to desire to lower the amplitude of the rear channel information but not the center channel information. The Madnick et al. device does not allow for this separate control. Further, there are no line level outputs to introduce additional amplification to the rear or center speakers to separate amplitude control of rear and center channels. The "bleeding" of the two channel input to drive five separate speakers has led to inadequate power response and frequency loss, most significantly in the center speaker. Although systems as seen in Madnick et al. have been adequate for reproduction of conversational audio, it has not successfully reproduced musical audio as felt and heard in film theaters.
There exists a need for an improved surround sound device allowing for multiple configuration of separate amplitude control of surround sound decoded output channels. The improved surround sound device needs to have a means for converting the speaker level signal to a line level signal so that additional amplification can be introduced into the system. Further, the device needs to employ passive circuitry for this level conversion to avoid introducing noise to the stereophonic signal.